Benthic–pelagic coupling: a comparison of the community structure of benthic and planktonic heterotrophic protists in shallow inlets of the southern Baltic

1. The taxonomic composition, abundance and biomass of heterotrophic protists (ciliates, heterotrophic flagellates (HF), rhizopods and actinopods) in the sediment and water column of shallow inlets of the Southern Baltic was studied under a variety of environmental conditions during 1996–1997. A shallow, highly eutrophic station and a deeper, less eutrophic station were compared.
2. Community biomass ranged from 0.12 to 0.34 μg C cm?3 in the water column and from 1.5 to 105 μg C cm?3 in the sediment. Heterotrophic protists dominated zooplankton biomass at both stations (73% and 84% mean contribution), while they were of minor importance within the zoobenthos. Expressed per unit area, benthic biomass contributed a significant part (44% and 49%) to the total heterotrophic protistan community at both stations.
3. Although the methodology for counting ciliates and HF was focussed on a high taxonomic resolution, the results reveal some general trends in the distribution of heterotrophic protists: protozooplankton biomass was dominated by flagellates (80% mean biomass contribution) at the shallow station and by ciliates (73% mean biomass contribution) at the deep station. In the benthos at both stations, ciliates were the dominant protozoans, followed by the hitherto little‐studied rhizopods (25% and 35% mean biomass contribution) and flagellates.
4. The degree of benthic–pelagic coupling differed between taxonomic groups. Benthic and pelagic communities of ciliates showed little taxonomic overlap. In contrast, many heterotrophic flagellate species were found both in the benthos and in the pelagic. These benthic–pelagic species contributed significantly to the biomass of HF in the water column. The planktonic rhizopod community consisted of a subset of those species found in the benthos.
5. The abundance of benthic and pelagic protists was positively correlated at the shallow station, but taxonomic data indicate that the direct exchange between benthic and pelagic communities was only partly responsible.     

Seasonal dynamics of phytoplankton in the Gulf of Aqaba,Red Sea

Seawater samples were collected biweekly from the northern Gulf of Aqaba, Red Sea, for Phytoplankton analysis during the period May 1998 to October 1999. Microscopic counts and HPLC methods were employed. Procaryotic and eucaryotic ultraplankton dominated throughout most of the year, with larger nano- and microplankton making up only 5% of the photosynthetic biomass. Moderate seasonal variations in the 0–125 m integrated Chl a contrasted with a pronounced seasonal succession of the major taxonomic groups, reflecting the changes in the density stratification of the water column: Prochlorococcus dominated during the stratified summer period and were almost absent in winter. Chlorophyceae and Cryptophyceae were dominant during winter mixing but scarce or absent during summer. Diatoms and Synechococcus showed sharp and moderate biomass peaks in late winter and spring respectively, but remained at only low Chl a levels for the rest of the year. Chrysophyceae, Prymnesiophyceae and the scarce Dinophyceae showed no clear seasonal distribution pattern. The implications of alternating procaryotic and eucaryote dominated algal communities for the Red Sea pelagic food web are discussed. Electronic Supplementary Material Electronic supplementary material is available for this article at and accessible for authorised users.     

Benthic–pelagic coupling in the population dynamics of the harmful cyanobacterium Microcystis

1. In eutrophic lakes, large amounts of the cyanobacterium Microcystis may overwinter in the sediment and re‐inoculate the water column in spring. 2. We monitored changes in pelagic and benthic populations of Microcystis in Lake Volkerak, The Netherlands. In addition, sedimentation rates and the rate of recruitment from the sediment were measured using traps. These data were used to model the coupling between the benthic and pelagic populations and to calculate the contribution of overwintering benthic and pelagic populations to the magnitude of the pelagic summer bloom. 3. Changes in the benthic Microcystis population showed a time lag of 3–14 weeks compared with the pelagic population. This time lag increased with lake depth. The largest amount of benthic Microcystis was found in the deepest parts of the lake. These observations suggest horizontal transport of sedimented Microcystis from shallow to deep parts of the lake. 4. Recruitment from and sedimentation to the sediment occurred throughout the year, with highest recruitment and sedimentation rates during summer. Model simulations indicate that the absence of benthic recruitment would reduce the summer bloom by 50%. 5. In spring, the total pelagic population was three to six times smaller than the total benthic population. Yet, model simulations predict that the absence of this small overwintering pelagic population would reduce the summer bloom by more than 64%. 6. Reduction of the overwintering pelagic populations, for instance by flushing, may be a useful management strategy to suppress or at least delay summer blooms of Microcystis.     

Microalgae biodiversity and biomass status in Qua Iboe Estuary mangrove swamp,Nigeria

Microalgae composition, abundance, diversity and biomass of the Qua Iboe Estuary mangrove swamp were studied. The results revealed the rich assemblage of the brackish ecosystem. Six major taxanomic classes were encountered. These were the Bacillariophyceae, Cyanophyceae, Chlorophyceae, Chrysophyceae, Euglenophyceae and Phaeophyceae. Their composition, abundance and diversity exhibits strong seasonal variation. Variations between pelagic and sedimentary habitats were also noticed. The diatoms (Bacillariophyceae) dominated the habitats. Actinoptychus undalatus, Navicula radiosa and Amphora ovalis co-dominated the pelagic water column; while the epipellic (intertidal) and benthic (subtidal) sediments were co-dominated by A. ovalis and Actinoptychus undulatus. Analyses of the Shannon’s index of general diversity (H¹), McArthur-Terborgh species equitability index (E) and Simpson’s index of dominance (D) of the microalgae communities revealed that A. ovalis with a mean H ¹ -value of 0.25, E-value of 0.08 and D-value of 0.05 was the most prevalent genus in the mangrove ecosystem despite its uneven distribution. The densities of the microalgae communities corresponded with their biomass statuses and were seriously impacted by oil spillage. This forms the basis of concern because the estuary is associated with a high probability of major oil pollution with serious consequences for ecological stability and fisheries.     

Top-down and bottom-up impacts of juvenile fish in a littoral reed stand

1. Research has often focused on pelagic food chains and processes of lakes; less is known about the contribution of benthic energy flows to whole‐lake ecosystem energetics. This stems from the fact that the shoreline and littoral habitats, which provide a key linkage between sediment and water column, have only recently become a significant focus for study. 2. This study aimed to quantify the feeding and phosphorus allocation of a juvenile fish community in a littoral zone of a shallow lake in response to the biomass succession of the invertebrate prey community. Habitats comprising reed and adjacent open water were sampled over two consecutive years during day and night. 3. Although there were substantial year‐to‐year differences in the biomass of invertebrates, the fish community composition, diet consumption rates and phosphorus allocations were very similar in both study years. Biomasses and predation impacts by juvenile fish on prey groups were substantially higher within the reeds than in the adjacent open water habitat. This may be explained by the refuge‐seeking behaviour of the fish. 4. In general, invertebrates were negligibly influenced by fish feeding, with the exception for a strong top‐down control of large cladocerans. In response to the resulting low Daphnia biomass, fish were forced to switch to a higher degree of benthivory. Consequently, juvenile fish in littoral reed stands may shift benthic‐derived energy and phosphorus via the excretion of soluble reactive phosphorus into the open water.     

The microphytobenthos of Königshafen—spatial and seasonal distribution on a sandy tidal flat

A microphytobenthic species composition of a tidal flat in the northern Wadden Sea was analysed regarding cell numbers and biomass (in carbon units). The three sampling sites differed in tidal inundation from 15 cm to about 90 cm water depth at high tide. The sediment was sandy at all three stations. A cluster analysis revealed a separation of the benthic diatoms into three areas: aNereis-Corophium-belt, a seagrass-bed and theArenicola-flat. Small epipsammic diatoms were most abundant and dominated the microalgal biomass. A microphytobenthic “spring bloom” even started beneath the ice cover of the flat in January. Lowest values of cell numbers and biomass of benthic microalgae were found in summer. Highest values were measured in the uppermost area (Nereis-Corophium-belt), and only here was an autumnal increase of benthic microalgae found. Further cluster analysis within each of the three areas revealed seasonal differences although the majority of species were present all year round. Many species were most abundant in spring, and some showed a bimodal distribution (spring-autumn) in the year of investigation.     

Tracing the life history of red gurnard (Aspitrigla cuculus) using validated otolith annual rings

Back calculation, marginal increment analysis and modal progression analysis were used to trace growth processes of the red gurnard, Aspitrigla cuculus L., in the Mediterranean Sea. Three false rings appeared regularly on otoliths and were hypothesized to refer to the ontogeny on the basis of the study of the recruitment and reproduction processes of the species. The first two rings were laid down during the first pelagic period and the transformation to the bottom mode of life, respectively. The third ring corresponded to the first spawning at the end of the second year of life. The marginal increment analysis demonstrated that one opaque and one translucent zone is formed each year, the translucent ring corresponding to the period of slow growth in the winter. Temperature and salinity of Mediterranean waters below the thermocline where red gurnard live do not undergo seasonal variations. Therefore, traditional interpretation of the discontinuity patterns on otoliths related to the decrease of water temperature should be rejected for Mediterranean fish distributed on the deep shelf and slope. Discontinuities in otolith structure may be related to seasonal variation in feeding intensity as a consequence of decreases in the benthic mysids biomass and time available to find food due to reduced daylight. Future research should focus on the trophic ecology of the demersal fishes and the seasonal fluctuations of the prey resources during the year in order to assess the role of the food supply with regard to otolith growth patterns in the Mediterranean Sea.     

Benthic resting periods of pelagic cyclopoids in an oligotrophic lake

The benthic resting stages of pelagic cyclopoids were studied in the oligotrophic lake Pääjärvi (maximum depth 87 m), southern Finland. Stage 5 copepodids of Thermocyclops oithonoides were found in the bottom from September to April, with highest abundances in the littoral at 1–2 m. Dormant Mesocyclops leuckarti (mainly stage 5 copepodids) were found from mid-August so April, with a strong concentration at the depth of 1.5 m, and resting stage 4 copepodids of Cyclops kolensis from mid-summer to late winter at the depths of 1–13 m. Single stage 4 and 5 copepodids of C. strenuus and C. lacustris were also found in the bottom during autumn and winter. Available information on the periodicity of these species conforms well with the hypothesis of a temperature-adjusted photoperiodic control of diapause.
In the sediment, the resting stages of pelagic cyclopoids were confined to the uppermost 2–3 cm, C. kolensis penetrating slightly deeper than the smaller Thermocyclops and Mesocyclops species.
Losses during the resting period were highest in T. oithonoides , in which the seasonal mortality rates were significantly correlated with temperature. Its overall mortality was slightly lower at 2 m than at 13 or 40 m. C. kolensis had the lowest mortality, with no clear relation to temperature.
The mean winter biomass of the benthic resting stages of pelagic cyclopoids exceeded that of the true benthic copepods, and the winter losses of the former were equal to about one fifth of the total annual production of the true benthic copepods.     

Modelling marine ecosystem response to climate change and trawling in the North Sea

The marine ecosystem response to climate change and demersal trawling was investigated using the coupled hydrodynamic-biogeochemical water column model GOTM-ERSEM-BFM for three contrasting sites in the North Sea. Climate change forcing was derived from the HadRM3-PPE-UK regional climate model for the UK for the period 1950–2100 using historical emissions and a medium emissions scenario (SRESA1B). Effects of demersal trawling were implemented as an additional mortality on benthic fauna, and changes in the benthic–pelagic nutrient and carbon fluxes. The main impacts of climate change were (i) a temperature-driven increase in pelagic metabolic rates and nutrient cycling, (ii) an increase in primary production fuelled by recycled nutrients, (iii) a decrease in benthic biomass due to increased benthic metabolic rates and decreased food supply as a result of the increased pelagic cycling, and (iv) a decrease in near-bed oxygen concentrations. The main impacts of trawling were (i) reduced benthic biomass due to the increased mortality, and (ii) the increased benthic–pelagic nutrient fluxes, with these effects counteracting each other, and relatively small changes in other variables. One important consequence was a large decrease in the de-nitrification flux predicted at the two summer-stratified sites because less benthic nitrate was available. The effects of trawling scaled linearly with fishing effort, with greatest sensitivity to fishing in summer compared to fishing in winter. The impacts of climate change and trawling were additive, suggesting little or no non-linear interactions between these disturbances.     

The contribution of epipelon to total sediment microalgae in a shallow temperate eutrophic loch (Loch Leven, Scotland)

Benthic microalgae are known to perform important ecosystem functions in shallow lakes. As such it is important to understand the environmental variables responsible for regulating community structure, positioning and biomass. We tested the hypothesis that the positioning (across a depth gradient of 2–22 m overlying water depth) and relative biomass (determined using bulk and lens tissue harvested chlorophyll (Chl) a concentrations) of the epipelon community would vary independently with season (12 monthly samples) and across natural gradients of light and habitat disturbance relative to the total benthic algal community (i.e. all viable microalgae in the surface sediments) in a shallow eutrophic loch. Total sediment microalgal Chl a concentrations (TS-Chl; range: 5–874 μg Chl a g⁻¹ dw) were highest in winter and in the deepest site (20 m overlying water depth), apparently as a result of phytoplanktonic settling and sediment focussing processes. Epipelic Chl a concentrations (Epi-Chl; range: <0.10–6.0 μg Chl a g⁻¹ dw) were highest in winter/spring, a period when water clarity was highest and TS-Chl lowest. Principal components analysis highlighted strong associations between Epi-Chl and sites of intermediate depths (2.5–5.5 m) in all seasons except autumn/winter. Autumn/winter represented the season with the highest average wind speeds preceding sampling, during which the highest Epi-Chl concentrations were associated with the deepest sites. Epi-Chl was associated with intermediate light and habitat disturbance during spring/summer and summer/autumn and varied positively with habitat disturbance, only, in autumn/winter and winter/spring. The epipelon community structure also varied with depth; diatoms dominated shallow water sediments, cyanobacteria dominated deep water sediments, and sediments at sites of intermediate depth returned the highest biovolume estimates and the most diverse communities. This study has strengthened the hypothesis that the structure and biomass of benthic microalgal communities in lakes are regulated by habitat disturbance and water clarity, both of which are expected to respond to climate change and eutrophication. The degree to which these structural responses reflect functional performance requires clarification.     

The role of microphytobenthos on shallow coastal lagoons: a modelling approach

Ria Formosa is a Region of Restricted Exchange given its limited connection to coastal water circulation. Furthermore, it is subject to several anthropogenic activities that can lead to an increase in nutrients and potentially to eutrophication. Previous studies have shown the importance of the benthic compartment, specifically the microphytobenthos (MPB) in this shallow coastal lagoon. The dCSTT–MPB model [new version of the dynamic Comprehensive Studies Task Team (dCSTT) model] here described couples the benthic and pelagic compartments. Due to the shallowness of the system, the benthic microalgae are one of the most important primary producers of the system. Preliminary results of the model show a large biomass of benthic microalgae, which strongly influences the pelagic chlorophyll concentration by resuspension. However, algae concentrations in the water column are relatively small due to the high flushing rate of the lagoon. The MPB community is mainly supported by nutrients in the pore water. A sensitivity analysis (SA) has revealed that the factors associated with the benthic compartment were the most important and sensitive to changes. Porosity, benthic chlorophyll recycling, loss of MPB due to grazing and the yield of microphytobenthic chlorophyll from nitrogen were some of the most sensitive parameters, as well as the ones associated with decay of particulate organic nitrogen. The development of our dCSTT–MPB model has itself provided insights into benthic function.     

Distribution patterns of fishes in a canyon-shaped reservoir

In August 2004 and 2005, an extensive study of the fish community was carried out in the largest water supply reservoir in the Czech Republic and Central Europe, the canyon‐shaped ?elivka Reservoir, using a fleet of Nordic multimesh gillnets. Fishes were sampled at eight locations along the longitudinal profile of the reservoir and at five benthic depth layers covering depths from the surface down to 18 m (benthic gillnet 1·5 m high), and at three pelagic depth layers down to the depth of 5 m above the bottom (pelagic gillnets 4·5 m high). Catches of both juvenile (age 0+ year) and adult (fishes >1 year) fishes were highest in the upper layers of the water column (i.e. in the epilimnion down to 5 m, and down to 10 m in the benthic habitats). Along the tributary–dam axis in the pelagic habitats, both juvenile and adult fishes preferred the upper part of the reservoir, where the maximum number of species and also the greatest abundance of zooplankton were found. In the benthic habitats, fishes selected location according to factors other than trophic status. More juvenile fishes were recorded in the benthic habitats than in the pelagic habitats. Depth had the largest explanatory power for predicting fish community composition, followed by the affiliation with benthic and pelagic habitats, and location on the longitudinal axis of the reservoir. The fish community was represented mainly by cyprinids and consisted of two distinct groups of species, with bleak Alburnus alburnus, rudd Scardinius erythrophthalmus and asp Aspius aspius dominating the offshore group while perch Perca fluviatilis and ruffe Gymnocephalus cernuus were affiliated with the inshore group of the adult fish community. Roach Rutilus rutilus, bream Abramis brama and pikeperch Sander lucioperca occurred in important proportions in both the inshore and the offshore zones. All species, with the exceptions of adult perch (1+ year and older), 0+ year perch and 0+ year roach, preferred the most eutrophic tributary part of the reservoir. The fish community was relatively stable between the 2 years sampled.     

Genetic Diversity in Microcystis Populations of a French Storage Reservoir Assessed by Sequencing of the 16S-23S rRNA Intergenic Spacer

We compared the genetic diversity of the 16S-23S spacer of the rRNA gene (ITS1) in benthic and pelagic colonies of the Microcystis genus isolated from two different sampling stations with different depths and at two different sampling times (winter and summer) in the French storage reservoir of Grangent. In all, 66 ITS1 sequences were found in the different clone libraries. The nucleotide diversity of all the sampled isolates were in the same range (average number = 0.022) regardless of their origin, showing that several clones are involved in the summer bloom event and contribute to the high biomass production. Phylogenetic study and analysis of molecular variance (AMOVA) revealed no obvious genetic differentiation between the benthic and pelagic isolates. This finding confirms that the Microcystis genus in this lake is characterized by having both a benthic phase in winter and spring allowing this organism to survive in unfavorable environmental conditions, and a pelagic phase in summer and autumn when environmental conditions allow them to grow in the water column. Finally, comparing these sequences with those available in the GenBank database showed that some highly conserved genotypes are found throughout the world.     

Seasonal changes of macrozooplankton and Benthic Boundary Layer macrofauna from the Bay of Saint-Brieuc (western English Channel)

The macrozooplankton and Benthic Boundary Layer (BBL) macrofauna over a coarse sand and pebble community in the Bay of Saint-Brieuc (western English Channel) were sampled with a WP2 zooplankton net and with a modified MACER-GIROQ suprabenthic sledge, respectively, from February 1994 to November 1995. One hundred and sixty-four species were collected in 44 suprabenthic sledge hauls and 19 taxa in 30 zooplankton net hauls. In the water column, appendicularians and cnidarians dominated, while, in the BBL, holoplanktonic amphipods, chaetognaths, amphipods and mysids dominated the fauna; among them Apherusa spp., Sagitta setosa Muller, Anchialina agilis (Sars), Sirella clausii Sars and Eusirus longipes Boeck were the dominant species. The density and biomass of the BBL macrozooplankton were lower than those of the macrozooplankton in the water column. The density and biomass of suprabenthos remained a low throughout the year. In the water column, density and biomass of macrozooplankton showed a maximum in spring and remained low from autumn to winter; conversely, in the BBL, the density and biomass of both macrozooplankton and suprabenthos were higher from summer to autumn. The change in abundance of both BBL and pelagic taxa was seasonal. Some species were primarily sampled in the water column (appendicularians, cladocerans and cnidarians), while others were preferentially found at the BBL (suprabenthic species, holoplanktonic amphipods and cephalopods). A third group was collected throughout the water column (chaetognaths and fish larvae).      

Hydrology Affects Environmental and Spatial Structuring of Microalgal Metacommunities in Tropical Pacific Coast Wetlands

The alternating climate between wet and dry periods has important effects on the hydrology and therefore on niche-based processes of water bodies in tropical areas. Additionally, assemblages of microorganism can show spatial patterns, in the form of a distance decay relationship due to their size or life form. We aimed to test spatial and environmental effects, modulated by a seasonal flooding climatic pattern, on the distribution of microalgae in 30 wetlands of a tropical dry forest region: the Pacific coast of Costa Rica and Nicaragua. Three surveys were conducted corresponding to the beginning, the highest peak, and the end of the hydrological year during the wet season, and species abundance and composition of planktonic and benthic microalgae was determined. Variation partitioning analysis (as explained by spatial distance or environmental factors) was applied to each seasonal dataset by means of partial redundancy analysis. Our results show that microalgal assemblages were structured by spatial and environmental factors depending on the hydrological period of the year. At the onset of hydroperiod and during flooding, neutral effects dominated community dynamics, but niche-based local effects resulted in more structured algal communities at the final periods of desiccating water bodies. Results suggest that climate-mediated effects on hydrology can influence the relative role of spatial and environmental factors on metacommunities of microalgae. Such variability needs to be accounted in order to describe accurately community dynamics in tropical coastal wetlands.     

Seasonal variability of meio- and macrobenthic standing stocks and diversity in an Arctic fjord (Adventfjorden, Spitsbergen)

Strong environmental seasonality is a basic feature of the Arctic system, still there are few published records of the seasonal variability of the Arctic marine biota. This study examined the year-round seasonal changes of soft bottom macro- and meiobenthic standing stocks and diversity on a station located in an Arctic fjord (Adventfjorden, Spitsbergen). The seasonality observed in benthic biota was related to the pelagic processes, primarily the seasonal fluxes of organic and inorganic particles. The highest abundance, biomass and richness of benthic fauna occurred in the spring after the phytoplankton bloom. During the summer, when a high load of glacial mineral material was transported to the fiord, the number of both meio- and macrobenthic individuals decreased remarkably. The strong inorganic sedimentation in summer was accompanied by a decline in macrobenthic species richness, but had no effects on evenness. Redundancy analysis (RDA) pointed to granulometric composition of sediments (depended on mineral sedimentation) and organic fluxes as factors best related to meio- and macrobenthic taxonomic composition, but no clear seasonal trend could be observed on the nMDS plots based on meiobenthic higher taxa or macrobenthic species abundances in the samples. This study addresses the possible effects of changes in the winter ice cover on the fjordic benthic systems because it was performed in a year with no ice cover on the fjord.     

Spatial and Temporal Variability in the Ecosystem Metabolism of a High-elevation Lake: Integrating Benthic and Pelagic Habitats

We characterized spatial and temporal variability in net ecosystem production (NEP), community respiration (CR), and gross primary production (GPP) over an ice-free season in an oligotrophic high-elevation lake using high-frequency measurements of dissolved oxygen. We combined the use of free-water and incubation chamber measurements to compare pelagic and benthic habitats and estimate their relative contributions to whole-lake metabolism. Despite a brief period of predominant heterotrophy after snowmelt, both free-water and incubation chamber measurements confirmed autotrophy of the epilimnion in all habitats throughout the ice-free season. In contrast, benthic incubation chambers showed the benthos to be consistently heterotrophic. Although temperature was the strongest seasonal driver of benthic metabolism, bacterioplankton density and indexes of organic matter quality explained the most variability in pelagic metabolism. Driven largely by benthic metabolism, free-water measurements of GPP and CR were twice as high in littoral than pelagic habitats. However, rates of water column primary production overlying the littoral benthos were high enough to overcome net benthic heterotrophy, and seasonal mean NEP in littoral habitats remained positive and not significantly different from pelagic habitats. Benthic rates averaged about 25% of whole-lake metabolism. Pelagic metabolism measurements were affected by littoral rates about half the time, with the degree of isolation between the two a function of advection and water column stability. These results emphasize the importance of characterizing spatial and temporal variability in metabolism within the context of physical dynamics and challenge the notion that benthic metabolism will necessarily be larger than pelagic metabolism in oligotrophic lakes.     

Top‐down and bottom‐up control of periphyton by benthivorous fish and light supply in two streams

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Influence of diet shifts in underyearling fish on phosphorus recycling in a hypertrophic biomanipulated reservoir

1. In addition to effects of direct predation by planktivorous fish, nutrient recycling by fish may also contribute to structuring foodwebs in lakes. There is little evidence, however, about whether underyearling fish undergoing several ontogenetic diet shifts may have a comparable bottom-up impact. 2. This study examined seasonal patterns of phosphorus (P) concentration and external load, phytoplankton, zooplankton and benthos, and diet shifts in three underyearling fish [perch (Perca fluviatilis), roach (Rutilus rutilus) and ruffe (Gymnocephalus cernuus)] in the shallow, hypertrophic biomanipulated Bautzen reservoir, Germany. Phosphorus metabolism of fish was calculated by a balanced bioenergetics model on the basis of fish diet, growth and water temperature. 3. The fish showed several shifts from planktivory to other food sources during the sampling period from May to September. These shifts were probably caused by the seasonal succession of the zooplankton community, mainly the midsummer decline of Daphnia galeata. 4. The diet shifts in fish also had consequences for the amount of P consumed and released. During periods of dominant zooplanktivory, the excretion of P did not exceed the removal of P stored in pelagic prey. By contrast, if benthivory dominated, fish subsidized the pelagic P pool by excreting more P from benthic prey than had been removed from the pelagic area. This occurred predominantly in perch and ruffe during periods of low zooplankton biomass, whereas the roach ate more algae and therefore excreted less P of benthic origin. 5. Phosphorus release by underyearling fish was estimated at a maximum of 0.1 mg m^–3 JY day^–1. This value was negligible compared with both the external load of P to Bautzen reservoir and the concentration of P in the pelagic area during summer. It is therefore concluded that both the predominance of underyearling zooplanktivorous fish and the high Daphnia biomass during certain periods of the year in the Bautzen reservoir may be the reason that nutrient release by the fish structured the foodweb only marginally. 6. This study suggests that biomanipulation has altered both top-down and bottom-up impacts of fish in Bautzen reservoir. The highest efficiency of foodweb manipulations may be obtained after reduction of the external P loading below a certain threshold. In turn, if external restoration of eutrophied lakes is not accompanied by changes in fish community, then the combined forces of strong zooplanktivory and high P recycling of dense stocks of zooplanktivorous and benthivorous fish may hold the water in a eutrophic-like stage, even if external load has been significantly reduced.     

Growth and tissue mass cycles in the infaunal bivalve Yoldia eightsi at Signy Island, Antarctica

Shell growth in Yoldia eightsi was measured over an austral summer and winter in 1992. In specimens < 12 mm length, growth was not significantly different between summer and winter periods, and the fastest recorded rate, 6.3 μm day⁻¹ was for 5-mm individuals during the winter. In summer, specimens of all lengths grew significantly, but in winter bivalves > 27 mm length did not increase in length. Tissue dry and ash-free dry mass (AFDM) cycles were assessed at monthly intervals between December 1988 and January 1991. ANCOVA indicated significant interannual and seasonal effects on this cycle. Tissue mass increased in the summer, coinciding with the phytoplankton bloom and the period of maximum sedimentation of organic material from the water column. A standard 20-mm-length animal reached a maximum AFDM of 114 mg in February 1990. The minimum value (68 mg AFDM) throughout the 2 years of measurements was in early December 1988, at the end of the austral winter. Periods of tissue mass increase were, therefore, decoupled from shell growth, at least in juveniles. Tissue mass was significantly higher in 1990 than 1989, which was mainly due to high organic contents in the summer (January to May). This was not consistent with the pattern of organic content in the sediments at the study site, but was in phase with the cycle in sediment chlorophyll a content. Tissue mass increase depended on major resource input during the summer, but Y. eightsi was capable of maintaining winter condition from stocks of benthic microalgae in years of poor ice cover. Tissue mass declined between April and July each year. This was accompanied by large falls in tissue ash content, and coincided with the spawning period in early June. These are the first monthly tissue mass data collected over a 2-year period for an Antarctic mollusc. They are the first such data indicating seasonal variation in tissue mass and showing a decoupling of shell and tissue growth in a polar bivalve. The P/B ratio calculated from these data was 0.106, which is slightly lower than previous values found for this species, but is in line with general values for Antarctic marine benthos. Accepted: 6 December 1999